Receivers have generally used an impulse noise blanker to reduce the audible effects of impulse noise in a signal, such as direct current (DC) motor noise, or other sources of impulsive electromagnetic interference (EMI). Examples of such receivers are amplitude modulated (AM) receivers and frequency modulated (FM) receivers. Typically, the impulse noise blanker systems include a detection circuit and a removal circuit.
In reference to FIG. 1, one example of a conventional noise blanker circuit is generally shown at reference indicator 10. The noise blanker circuit 10 includes a detection circuit generally indicated at reference indicator 12 and a removal circuit generally indicated at reference indicator 14. The detection circuit 12 has a complex intermediate frequency (IF) input 16 that is separated into a radio frequency (RF) level input 18 and an ultrasonic noise (USN) input 20. A peak-to-average detector 22 is applied to the RF level input 18, and a peak-to-average detector 24 is applied to the USN input 20. If either of the peak-to-average detectors 22,24 detects impulse noise by detecting a large peak-to-average ratio, then the removal circuit 14 is activated in order to remove the impulse noise. Thus, the detection circuit 12 distinguishes impulse noise from the desired signal and other types of noise.
The detection circuit 12 can also include a high-pass filter 26 for the RF level input 18 and a high-pass filter 28 for the USN input 20. Generally, the high-pass filters 26,28 separate all of the noise in the signal from the desired portion of the signal. The peak-to-average detectors 22,24 separate the impulse noise from the other types of noise in the signal. Thus, the detection circuit 12 can detect impulse noise in order to activate the removal circuit 14.
The removal circuit 14 generally includes a delay device 30 and a pulse removal device 32. The delay device 30 is typically used to match in time the input signal from an FM demodulator 34 with the output from a blank pulse generator 36. Thus, the delay device 30 can delay the input signal from the FM demodulator 34 in order to match the processing time of the detection circuit 12 and the output of the blank pulse generator 36. The blank pulse generator 36 outputs a blank pulse when impulse noise is detected. The removal circuit 14 then removes the impulse noise from the signal based upon the output from the blank pulse generator 36 and emits an output.
Typically, impulse noise blanking is generally most effective if there is little or no filtering before the noise blanker circuit 10 because of the inverse relationship between the bandwidth of a filter and the filter's impulse response. Thus, the narrower the bandwidth of the filter, the longer in time the filter's impulse response. By increasing the time of the impulse response of the filter, it is generally more difficult to remove an impulse noise in the output of the filter. Further, algorithms that include variable bandwidth IF filters and channel equalizers result in stretching the impulse noise over time. Additionally, removal of the impulse noise can result in a different type of undesirable audible noise in the signal.
Generally, a sample and hold circuit can be used to remove the impulse noise in a signal when the blanking or removal is performed after the signal is demodulated. Alternatively, when the signal is at IF, and before the signal is demodulated, the impulse noise in a signal can be removed, but an oscillator is usually required to maintain a constant frequency of the signal at IF.
However, receivers have been developed that include an analog-to-digital (A/D) converter at the IF. With the addition of the AID converter, additional algorithms are required to remove the impulse noise, which typically are difficult and/or costly to implement in an analog receiver. For example, the additional algorithms, in conjunction with the limited bandwidth of the AID converter, have typically resulted in the noise blanking circuit 10 being inadequate.
Therefore, it is desirable to develop a circuit and method for noise blanking in a digitized system, where the impulse noise can be effectively detected, removed, and corrected.